%% @doc erlang_migrate — database migration library modeled after golang-migrate/v4.
%%
%% == Quick Start ==
%% ```
%% Config = #{
%%     conn  => Conn,           % epgsql connection pid
%%     dir   => "priv/migrations",
%%     table => <<"schema_migrations">>,  % optional
%%     lock_id => 7369284       % optional, auto-derived from table name
%% },
%% ok = erlang_migrate:up(Config).
%% '''
%%
%% == File naming ==
%% Migrations live in `dir' as pairs:
%%   {version}_{title}.up.sql
%%   {version}_{title}.down.sql   (optional — skipping makes down/2 fail)
%%
%% version must be a positive integer, e.g. 1, 100, 20240101120000.
%%
%% == GracefulStop ==
%% Send `erlang_migrate_abort' to the migration process to abort between
%% migrations:  MigPid ! erlang_migrate_abort
%%
%% == dry_run mode ==
%% Set `dry_run => true' in Config to log what would be applied without
%% touching the database.
-module(erlang_migrate).
-export([up/1, up/2, down/1, down/2, goto/2, version/1, force/2, drop/1]).

-define(DEFAULT_TABLE,              <<"schema_migrations">>).
-define(DEFAULT_LOCK_TIMEOUT,       15000).  %% milliseconds
-define(DEFAULT_SET_VERSION_RETRIES, 3).
-define(DEFAULT_SET_VERSION_RETRY_MS, 200).

%% Apply all pending migrations.
-spec up(Config :: map()) -> ok | {error, term()}.
up(Config) -> up(Config, all).

%% Apply up to N pending migrations (all = unlimited).
-spec up(Config :: map(), Steps :: pos_integer() | all) -> ok | {error, term()}.
up(Config, Steps) ->
    DryRun = dry_run(Config),
    with_lock(Config, fun(Conn, Table, Logger, Driver) ->
        case check_dirty(Driver, Conn, Table) of
            {error, _} = E -> E;
            ok ->
                {ok, Current, _} = Driver:current_version(Conn, Table),
                case erlang_migrate_source:scan(dir(Config)) of
                    {error, _} = E -> E;
                    {ok, All} ->
                        Pending = pending_up(All, Current, Steps),
                        apply_up(Driver, Conn, Table, Pending, Logger, DryRun, Config)
                end
        end
    end).

%% Roll back all applied migrations.
-spec down(Config :: map()) -> ok | {error, term()}.
down(Config) -> down(Config, all).

%% Roll back N migrations (all = unlimited).
-spec down(Config :: map(), Steps :: pos_integer() | all) -> ok | {error, term()}.
down(Config, Steps) when Steps =:= all orelse (is_integer(Steps) andalso Steps > 0) ->
    DryRun = dry_run(Config),
    with_lock(Config, fun(Conn, Table, Logger, Driver) ->
        case check_dirty(Driver, Conn, Table) of
            {error, _} = E -> E;
            ok ->
                {ok, Current, _} = Driver:current_version(Conn, Table),
                case Current of
                    undefined -> ok;
                    _ ->
                        case erlang_migrate_source:scan(dir(Config)) of
                            {error, _} = E -> E;
                            {ok, All} ->
                                ToRollback = pending_down(All, Current, Steps),
                                apply_down(Driver, Conn, Table, ToRollback, Logger, DryRun, Config)
                        end
                end
        end
    end).

%% Migrate to a specific version (auto up or down).
-spec goto(Config :: map(), Version :: integer()) -> ok | {error, term()}.
goto(Config, Version) ->
    DryRun = dry_run(Config),
    with_lock(Config, fun(Conn, Table, Logger, Driver) ->
        case check_dirty(Driver, Conn, Table) of
            {error, _} = E -> E;
            ok ->
                {ok, Current, _} = Driver:current_version(Conn, Table),
                case erlang_migrate_source:scan(dir(Config)) of
                    {error, _} = E -> E;
                    {ok, All} ->
                        CurV = case Current of undefined -> 0; V -> V end,
                        %% Fix #15: use dropwhile/takewhile for O(range) traversal
                        %% instead of full-list comprehension with two maps:get per element.
                        if
                            Version > CurV ->
                                Pending = lists:takewhile(
                                    fun(M) -> maps:get(version, M) =< Version end,
                                    lists:dropwhile(fun(M) -> maps:get(version, M) =< CurV end, All)),
                                apply_up(Driver, Conn, Table, Pending, Logger, DryRun, Config);
                            Version < CurV ->
                                Range = lists:takewhile(
                                    fun(M) -> maps:get(version, M) =< CurV end,
                                    lists:dropwhile(fun(M) -> maps:get(version, M) =< Version end, All)),
                                apply_down(Driver, Conn, Table, lists:reverse(Range), Logger, DryRun, Config);
                            true ->
                                ok
                        end
                end
        end
    end).

%% Return current schema version and dirty flag.
-spec version(Config :: map()) -> {ok, integer() | undefined, boolean()} | {error, term()}.
version(Config) ->
    Conn   = conn(Config),
    Table  = table(Config),
    Driver = driver(Config),
    case Driver:ensure_table(Conn, Table) of
        {error, _} = E -> E;
        ok ->
            case Driver:current_version(Conn, Table) of
                {ok, Ver, Dirty} -> {ok, Ver, Dirty};
                Err              -> Err
            end
    end.

%% Force set version (clears dirty flag — use after manual recovery).
%% Fix #6: validate Version exists in source files when 'dir' is configured.
%% Pass Version=undefined to clear the tracking table (reset to no migrations applied).
-spec force(Config :: map(), Version :: integer() | undefined) -> ok | {error, term()}.
force(Config, Version) ->
    Conn   = conn(Config),
    Table  = table(Config),
    Driver = driver(Config),
    case Driver:ensure_table(Conn, Table) of
        {error, _} = E -> E;
        ok ->
            case validate_force_version(Config, Version) of
                {error, _} = E -> E;
                ok -> Driver:set_version(Conn, Table, Version, false)
            end
    end.

validate_force_version(_Config, undefined) -> ok;
validate_force_version(Config, Version) when is_integer(Version) ->
    case maps:find(dir, Config) of
        error ->
            ok;  %% no dir configured — skip validation (backward-compatible)
        {ok, Dir} ->
            case erlang_migrate_source:scan(Dir) of
                {error, _} = E -> E;
                {ok, All} ->
                    Known = [maps:get(version, M) || M <- All],
                    case lists:member(Version, Known) of
                        true  -> ok;
                        false -> {error, {unknown_version, Version, Known}}
                    end
            end
    end.

%% Drop schema_migrations table (destructive — use in tests only).
-spec drop(Config :: map()) -> ok | {error, term()}.
drop(Config) ->
    Conn   = conn(Config),
    Table  = table(Config),
    Driver = driver(Config),
    Driver:drop_table(Conn, Table).

%%% Internal helpers

with_lock(Config, Fun) ->
    Conn    = conn(Config),
    Table   = table(Config),
    LockId  = lock_id(Config, Table),
    Timeout = lock_timeout(Config),
    Logger  = logger(Config),
    Driver  = driver(Config),
    case Driver:ensure_table(Conn, Table) of
        {error, _} = E -> E;
        ok ->
            log(Logger, info, #{}, fmt("acquiring lock ~b (timeout ~bms)", [LockId, Timeout])),
            case Driver:lock(Conn, LockId, Timeout) of
                {error, lock_timeout} ->
                    log(Logger, error, #{}, fmt("lock timeout after ~bms", [Timeout])),
                    {error, lock_timeout};
                {error, _} = E ->
                    log(Logger, error, #{}, <<"lock acquisition failed">>),
                    E;
                ok ->
                    log(Logger, info, #{}, <<"lock acquired">>),
                    try Fun(Conn, Table, Logger, Driver)
                    after
                        Driver:unlock(Conn, LockId),
                        log(Logger, info, #{}, <<"lock released">>)
                    end
            end
    end.

check_dirty(Driver, Conn, Table) ->
    case Driver:is_dirty(Conn, Table) of
        {ok, true}  -> {error, {dirty_state, "Run force/2 to recover"}};
        {ok, false} -> ok;
        Err         -> Err
    end.

pending_up(All, undefined, all) -> All;
pending_up(All, undefined, N)   -> lists:sublist(All, N);
pending_up(All, Current, all)   -> [M || M <- All, maps:get(version, M) > Current];
pending_up(All, Current, N)     -> lists:sublist([M || M <- All, maps:get(version, M) > Current], N).

pending_down(All, Current, all) ->
    lists:reverse([M || M <- All, maps:get(version, M) =< Current]);
pending_down(All, Current, N) ->
    lists:sublist(lists:reverse([M || M <- All, maps:get(version, M) =< Current]), N).

%% Fix #3 (GracefulStop): check own mailbox for an abort signal between migrations.
check_abort() ->
    receive
        erlang_migrate_abort -> {error, aborted}
    after 0 -> ok
    end.

apply_up(_Driver, _Conn, _Table, [], _Logger, _DryRun, _Config) -> ok;
apply_up(Driver, Conn, Table, [M | Rest], Logger, DryRun, Config) ->
    %% Fix #3: honour abort signal sent from another process
    case check_abort() of
        {error, aborted} -> {error, aborted};
        ok ->
            Version = maps:get(version, M),
            log(Logger, info, #{version => Version, title => maps:get(title, M)},
                fmt("applying up ~b ~s", [Version, maps:get(title, M)])),
            case run_one_up(Driver, Conn, Table, Version, maps:get(up_file, M),
                            Logger, DryRun, Config) of
                ok ->
                    log(Logger, info, #{version => Version}, fmt("applied up ~b", [Version])),
                    apply_up(Driver, Conn, Table, Rest, Logger, DryRun, Config);
                {error, _} = E -> E
            end
    end.

run_one_up(Driver, Conn, Table, Version, UpFile, Logger, DryRun, Config) ->
    case erlang_migrate_source:read_sql(UpFile, up) of
        {error, _} = E -> E;
        {ok, SQL} ->
            %% Fix #12: dry_run skips exec_sql and set_version entirely
            if DryRun ->
                log(Logger, info, #{version => Version, dry_run => true},
                    fmt("[dry-run] would apply up ~b", [Version])),
                ok;
            true ->
                case Driver:set_version(Conn, Table, Version, true) of
                    {error, _} = E -> E;
                    ok ->
                        case Driver:exec_sql(Conn, SQL) of
                            {error, _} = E ->
                                log(Logger, error, #{version => Version},
                                    fmt("failed up ~b — dirty state set", [Version])),
                                E;
                            ok ->
                                %% Fix #2: retry set_version(false) on transient failures
                                set_version_with_retry(Driver, Conn, Table, Version, false,
                                                       set_version_retries(Config),
                                                       set_version_retry_ms(Config),
                                                       Logger)
                        end
                end
            end
    end.

apply_down(_Driver, _Conn, _Table, [], _Logger, _DryRun, _Config) -> ok;
apply_down(Driver, Conn, Table, [M | Rest], Logger, DryRun, Config) ->
    case check_abort() of
        {error, aborted} -> {error, aborted};
        ok ->
            Version  = maps:get(version, M),
            DownFile = maps:get(down_file, M),
            case DownFile of
                undefined -> {error, {no_down_migration, Version}};
                _ ->
                    log(Logger, info, #{version => Version, title => maps:get(title, M)},
                        fmt("applying down ~b ~s", [Version, maps:get(title, M)])),
                    PrevVersion = case Rest of
                        []         -> undefined;
                        [Next | _] -> maps:get(version, Next)
                    end,
                    case run_one_down(Driver, Conn, Table, Version, PrevVersion, DownFile,
                                     Logger, DryRun, Config) of
                        ok ->
                            log(Logger, info, #{version => Version}, fmt("applied down ~b", [Version])),
                            apply_down(Driver, Conn, Table, Rest, Logger, DryRun, Config);
                        {error, _} = E -> E
                    end
            end
    end.

run_one_down(Driver, Conn, Table, Version, PrevVersion, DownFile, Logger, DryRun, Config) ->
    case erlang_migrate_source:read_sql(DownFile, down) of
        {error, _} = E -> E;
        {ok, SQL} ->
            if DryRun ->
                log(Logger, info, #{version => Version, dry_run => true},
                    fmt("[dry-run] would apply down ~b", [Version])),
                ok;
            true ->
                case Driver:set_version(Conn, Table, Version, true) of
                    {error, _} = E -> E;
                    ok ->
                        case Driver:exec_sql(Conn, SQL) of
                            {error, _} = E ->
                                log(Logger, error, #{version => Version},
                                    fmt("failed down ~b — dirty state set", [Version])),
                                E;
                            ok ->
                                set_version_with_retry(Driver, Conn, Table, PrevVersion, false,
                                                       set_version_retries(Config),
                                                       set_version_retry_ms(Config),
                                                       Logger)
                        end
                end
            end
    end.

%% Fix #2: retry set_version on transient failure, e.g. network flash after exec_sql succeeds.
set_version_with_retry(Driver, Conn, Table, Version, Dirty, 0, _RetryMs, _Logger) ->
    case Driver:set_version(Conn, Table, Version, Dirty) of
        ok           -> ok;
        {error, _} = E -> E
    end;
set_version_with_retry(Driver, Conn, Table, Version, Dirty, Retries, RetryMs, Logger) ->
    case Driver:set_version(Conn, Table, Version, Dirty) of
        ok -> ok;
        {error, _} ->
            log(Logger, warning, #{version => Version, retries_left => Retries},
                fmt("set_version retry (~b left) for ~b", [Retries, if Version =:= undefined -> 0; true -> Version end])),
            timer:sleep(RetryMs),
            set_version_with_retry(Driver, Conn, Table, Version, Dirty, Retries - 1, RetryMs, Logger)
    end.

conn(#{conn := C})   -> C.
table(#{table := T}) -> T;
table(_)             -> ?DEFAULT_TABLE.
dir(#{dir := D})     -> D.

dry_run(#{dry_run := true}) -> true;
dry_run(_)                  -> false.

set_version_retries(#{set_version_retries := N}) when is_integer(N), N >= 0 -> N;
set_version_retries(_) -> ?DEFAULT_SET_VERSION_RETRIES.

set_version_retry_ms(#{set_version_retry_ms := MS}) when is_integer(MS), MS >= 0 -> MS;
set_version_retry_ms(_) -> ?DEFAULT_SET_VERSION_RETRY_MS.

lock_id(#{lock_id := Id}, _) -> Id;
lock_id(_, Table)            -> erlang:phash2(Table, 1 bsl 30).

lock_timeout(#{lock_timeout := T}) when is_integer(T), T >= 0 -> T;
lock_timeout(_)                                                -> ?DEFAULT_LOCK_TIMEOUT.

logger(#{logger := F}) when is_function(F, 2) -> F;
logger(#{logger := F}) when is_function(F, 3) -> F;
logger(_)                                     -> undefined.

%% Fix #16: support both fun(Level, Msg) and fun(Level, Meta, Msg) loggers.
%% Meta is a map carrying structured context (version, title, dry_run, etc.).
log(undefined, _Level, _Meta, _Msg) -> ok;
log(Fun, Level, _Meta, Msg) when is_function(Fun, 2) -> Fun(Level, Msg);
log(Fun, Level, Meta, Msg)  when is_function(Fun, 3) -> Fun(Level, Meta, Msg).

%% Raises error/1 for bad driver config — these are programmer errors caught at startup.
driver(#{driver := D}) when is_atom(D) ->
    case code:which(D) of
        non_existing -> error({unknown_driver, D});
        _            -> D
    end;
driver(#{driver := D}) -> error({invalid_driver, D});
driver(_)              -> erlang_migrate_pg.

fmt(Fmt, Args) -> iolist_to_binary(io_lib:format(Fmt, Args)).